The Zip 100 Plus (SCSI + Parallel) is more problematic than the traditional Zip 100 SCSI and may be the source of your problems.
Looking at reviews from the 1990s, I gather that you are supposed to use the Autodetect connector when connecting to a SCSI port or a PC parallel port. The Zip drive then performs some clever logic to determine which interface is being used on the host, which possibly does not occur if you connect to the SCSI pass through port. So try plugging into Autodetect with an appropriate SCSI address.
Does the Zip 100 Plus have a SCSI bus termination switch? If so it should be on, but an external terminator may help if it is not provided. Slow SCSI devices are generally unfussy about termination.
On the Mac install a suitable version of SCSI probe -- it will tell you whether the Zip 100 Plus is recognised as a SCSI device.
If you want to add more devices to the SCSI chain, get rid of the Zip 100 Plus and buy a Zip 100 SCSI -- you will save a lot of time and frustration.
Looking at reviews from the 1990s, I gather that you are supposed to use the Autodetect connector when connecting to a SCSI port or a PC parallel port. The Zip drive then performs some clever logic to determine which interface is being used on the host, which possibly does not occur if you connect to the SCSI pass through port. So try plugging into Autodetect with an appropriate SCSI address.
Does the Zip 100 Plus have a SCSI bus termination switch? If so it should be on, but an external terminator may help if it is not provided. Slow SCSI devices are generally unfussy about termination.
On the Mac install a suitable version of SCSI probe -- it will tell you whether the Zip 100 Plus is recognised as a SCSI device.
If you want to add more devices to the SCSI chain, get rid of the Zip 100 Plus and buy a Zip 100 SCSI -- you will save a lot of time and frustration.